arXiv subjects
Publications and source records attributed to L. Cosmai.
We present results for the chromo-electric field generated by a static quark-antiquark pair at nearly zero temperature in lattice QCD with 2+1 dynamical staggered fermions at physical quark masses. We investigate the evolution of the flux-tube structure as the distance between the static color charges increases. We find hints that string breaking occurs at a distance in the range $0.963 \; \text{fm} \; \lesssim \; d^* \lesssim \; 1.156 \; \text{fm}$.
We present a study of the structure of the chromoelectrical field created by a static quark-antiquark pair in lattice QCD with 2+1 flavours of dynamical quarks, where the quark masses are set to their physical values. The analysis covers a wide range of temperatures both above and below the chiral crossover, and explores varying quark-antiquark distances, with the aim of identifying signals of deconfinement and string breaking in the field structure. To this end we apply the zero-curl perturbative field subtraction method, developed in our earlier studies of pure gauge SU(3) theory and of full QCD at zero temperature.
A recent paper has criticised the idea that, beside the resonance of mass $m_h= 125$ GeV, the Higgs field might exhibit a relatively narrow, second resonance with a mass $M_H \sim 690$ GeV. Without considering the evidence we have provided, the criticism also concerned our claim that experimental signals for this new resonance might already be seen in some LHC data. Since our extensive work is covered by several papers, we will summarise here the whole issue, namely: i) the theoretical motivations for a two-mass structure in cutoff $\Phi^4$ theory; ii) the checks from lattice simulations and the prediction $(M_H)^{\rm Theor} = 690\,(30)$ GeV; iii) the present experimental indications of a new, relatively narrow resonance in the expected mass range. This compact presentation will thus give the elements to objectively judge on a relevant question of present-day particle physics.
Spontaneous symmetry breaking through the Higgs field has been experimentally confirmed as a basic ingredient of the Standard Model. However, the origin of the phenomenon may not be entirely clear, because, in perturbation theory, the vacuum turns out to be a metastable state. An alternative scenario was proposed that implies a second resonance of the Higgs field ${\cal H}$ with a well delimited mass $(M_H)^{\rm Theor} = 690\,(30)$ GeV. This stabilises the potential, but, owing to an $H$ coupling to longitudinal $W$s with the same typical strength as that of the low-mass state with $m_h= 125$ GeV, it would still remain a relatively narrow resonance. Our scope here is twofold. First, leaving out many details, we outline a simple logical path where the, apparently surprising, idea of such a second resonance follows from basic properties of $\Phi^4$ theories. Secondly, we spell out a definite experimental signature of this resonance that is clearly visible in various LHC data. As a by-product, the ${\cal H} ^3$ term gives $\kappa_\lambda = (M_H/m_h) \sim $ 5.5 consistently with the ATLAS and CMS data.
An alternative to the idea of a metastable electroweak vacuum would be an initial restriction to the pure scalar sector of the Standard Model, but describing spontaneous symmetry breaking consistently with studies indicating that there are two different mass scales in the problem: a mass scale $M_H$ associated with the zero-point energy and a mass scale $m_h$ defined by the quadratic shape of the potential at its minimum. Therefore, differently from perturbation theory where these two mass scales coincide, the Higgs field could exhibit a second resonance with mass $(M_H)^{\rm Theor} = 690\,(30)$ GeV. This stabilises the potential, but the heavy Higgs $H$ would couple to longitudinal $W$s with the same typical strength as the low-mass state with $m_h=125$ GeV and so would still remain a relatively narrow resonance. While interesting signals from LHC experiments were previously pointed out, we have now enlarged our data sample, sharpened the analysis of some final states, and noted correlations between different channels that point directly to such a second resonance. The combined statistical evidence, even if roughly estimated, is thus so large that the observed deviations from the background cannot represent statistical fluctuations.
We present lattice Monte Carlo results on the chromoelectric field created by a static quark-antiquark pair in the vacuum of QCD with 2+1 dynamical staggered fermions at physical masses. After isolating the nonperturbative, confining part of the field, we characterize its spatial profile for several values of the physical distances between the sources, ranging from about 0.5 fm up to the onset of string breaking. Moreover, we compare our results with a model of QCD vacuum as disordered chromomagnetic condensate.
We report on the results of measuring the chromoelectric fields in a flux tube created by a static quark-antiquark pair in the finite-temperature SU(3) gauge theory. Below the deconfinement temperature the field behavior is similar to the zero-temperature case. Above the deconfinement temperature the field shape remains the same, but the field values drop when the distance between quark and antiquark increases, thus showing the disappearance of confining potential.
A characteristic signature of quark confinement is the concentration of the chromoelectric field between a static quark-antiquark pair in a flux tube. However, the structure of this flux tube, and hence of the confining force, has not been completely understood. Here we perform new lattice measurements of field distributions on smeared Monte Carlo ensembles in SU(3) gauge theory. On the basis of these simulations we demonstrate that the confining force can be understood using the analogy with the basic principles of electromagnetism as elucidated by Maxwell. We derive a chromomagnetic Lorentz force density coupling the chromoelectric field to chromomagnetic currents and integrate this force density over the flux tube interior to obtain a Maxwell-like force that squeezes the flux tube in the transverse direction. We show that the strength of this transverse confining force is equal to the value of the string tension calculated numerically from the chromoelectric field on the midplane between the quarks, verifying the consistency of these two complementary pictures of confinement.
We measure the spatial distribution of all components of the color fields surrounding a static quark antiquark pair in QCD with (2+1) HISQ flavors. We isolate the nonperturbative component of the longitudinal chromoelectric color field responsible for the linear term in the confining potential.
We extend a previous numerical study of SU(3) Yang-Mills theory in which we measured the spatial distribution of all components of the color fields surrounding a static quark-antiquark pair for a wide range of quark-antiquark separations, and provided evidence that the simulated gauge invariant chromoelectric field can be separated into a Coulomb-like 'perturbative' field and a 'non-perturbative' field, identified as the confining part of the SU(3) flux tube field. In this paper we hypothesize that the fluctuating color fields not measured in our simulations do not contribute to the string tension. Under this assumption the string tension is determined by the color fields we measure, which form a tensor $F_{μν}$ pointing in a single direction in color space. We call this the Maxwell mechanism of confinement. We provide an additional procedure to isolate the non-perturbative (confining) field. We then extract the string tension from a stress energy-momentum tensor $T_{μν}$ having the Maxwell form, constructed from the non-perturbative part of the tensor $F_{μν}$ obtained from our simulations. To test our hypothesis we calculate the string tension from our simulations of the color fields for ten values of the quark-antiquark separation ranging from 0.37 fm to 1.2 fm. We also calculate the spatial distributions of the energy-momentum tensor $T_{μν}$ surrounding static quarks for this range of separations, and we compare these distributions with those obtained from direct simulations of the energy-momentum tensor in SU(3) Yang-Mills theory.
Using lattice Monte Carlo simulations of SU(3) pure gauge theory, we determine the spatial distribution of all components of the color fields created by a static quark and antiquark. We identify the components of the measured chromoelectric field transverse to the line connecting the quark-antiquark pair with the transverse components of an effective Coulomb-like field $\vec{E}^C $ associated with the quark sources. Subtracting $\vec{E}^C$ from the total simulated chromoelectric field $\vec{E}$ yields a non-perturbative, primarily longitudinal chromoelectric field $\vec{E}^{NP}$, which we identify as the confining field. This is the first time that the chromoelectric field has been separated into perturbative and nonperturbative components, creating a new tool to study the color field distribution between a quark and an antiquark, and thus the long distance force between them.
We investigate the transverse profile of the chromoelectric field generated by a quark-antiquark pair in the vacuum of (2+1) flavor QCD. Monte Carlo simulations are performed adopting the HISQ/tree action discretization, as implemented in the publicly available MILC code, suitably modified to measure the chromoelectric field. We work on the line of constant physics, with physical strange quark mass $m_s$ and light to strange mass ratio $m_l/m_s = 1/20$.
In the present paper we consider $f(R)$ gravity theories in the metric approach and we derive the equations of motion, focusing also on the boundary conditions. In such a way we apply the general equations to a first order perturbation expansion of the Lagrangian. We present a model able to fit supernovae data without introducing dark energy.
The beginning of the cosmological phase bearing the direct kinematic imprints of supernovae dimming may significantly vary within different models of late-time cosmology, even if such models are able to fit present SNe data at a comparable level of statistical accuracy. This effect -- useful in principle to discriminate among different physical interpretations of the luminosity-redshift relation -- is illustrated here with a pedagogical example based on the LTB geometry.
We exploit analytic continuation to prolongate to the region of real chemical potentials the (pseudo)critical lines of QCD with two degenerate staggered fermions at nonzero temperature and quark or isospin density obtained in the region of imaginary chemical potentials. We determine the curvatures at zero chemical potential and quantify the deviation between the cases of finite quark and of finite isospin chemical potential. In both circumstances deviations from a quadratic dependence of the pseudocritical lines on the chemical potential are clearly seen. We try different extrapolations and, for the nonzero isospin chemical potential, confront them with the results of direct Monte Carlo simulations. We also find that, as for the finite quark chemical potential, an imaginary isospin chemical potential can strengthen the transition till turning it into strong first order.
We determine the (pseudo)critical lines of QCD with two degenerate staggered fermions at nonzero temperature and quark or isospin density, in the region of imaginary chemical potentials; analytic continuation is then used to prolongate to the region of real chemical potentials. We obtain an accurate determination of the curvatures at zero chemical potential, quantifying the deviation between the case of finite quark and of finite isospin chemical potential. Deviations from a quadratic dependence of the pseudocritical lines on the chemical potential are clearly seen in both cases: we try different extrapolations and, for the case of nonzero isospin chemical potential, confront them with the results of direct Monte Carlo simulations. Finally we find that, as for the finite quark density case, an imaginary isospin chemical potential can strengthen the transition till turning it into strong first order.
We present preliminary results about the critical line of QCD with two degenerate staggered quarks at nonzero temperature and chemical potential, obtained by the method of analytic continuation. As in our previous studies with different numbers of colors and flavors, we find deviations from a simple quadratic dependence on the chemical potential. We comment on the shape of the critical line at real chemical potential and give an estimate of the curvature of the critical line, both for quark chemical potential and isospin chemical potential.
We further elaborate on our proposal for the Trivial Higgs that within the Standard Model is the unique possibility to implement the spontaneous symmetry breaking of the local gauge symmetry by elementary local scalar fields. The Trivial Higgs boson turns out to be rather heavy with mass $m_H \simeq 750$ GeV. We discuss the experimental signatures of our Trivial Higgs and compare with the recent data from ATLAS and CMS collaborations based on a total integrated luminosity between 1 fb$^{-1}$ and 2.3 fb$^{-1}$. We suggest that the available experimental data could be consistent with our scenario.